You can just use the palm of your hand over your mouth,
however to determine if you you are making progress, you make like a meter to measure your performance.
A Spirometer is an apparatus for measuring the Volume of air Inspired and Expired by the lungs : https://en.m.wikipedia.org/wiki/Spirometer
What Is an Incentive Spirometer? : https://www.webmd.com/lung/lung-incentive-spirometer
Edit add; the palm of